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Abstract

We have developed a single-shot terahertz time-domain spectrometer to perform optical-pump/terahertz-probe experiments in pulsed, high magnetic fields up to 30 T. The single-shot detection scheme for measuring a terahertz waveform incorporates a reflective echelon to create time-delayed beamlets across the intensity profile of the optical gate beam before it spatially and temporally overlaps with the terahertz radiation in a ZnTe detection crystal. After imaging the gate beam onto a camera, we can retrieve the terahertz time-domain waveform by analyzing the resulting image. To demonstrate the utility of our technique, we measured cyclotron resonance absorption of optically excited carriers in the terahertz frequency range in intrinsic silicon at high magnetic fields, with results that agree well with published values.

Figures (7)

Schematic diagram of the single-shot detection scheme. Two 10× telescopes, L1-L4, are used to expand the optical gate beam so that a relatively uniform intensity profile reflects off of the echelon mirror. After encoding time delay information onto the intensity profile of the gate beam with the echelon optic, the intensity profile at the plane of the echelon surface is imaged onto the electro-optic sampling crystal after reflecting off of a pellicle beam splitter, PBS, overlapping with the THz beam and finally onto a CCD camera with image relay optics, L5-L8. A Wollaston prism, WP, is used to separate the two orthogonal polarizations of the elliptically polarized gate beam after a quarter-wave plate, QWP, and a cylindrical lens, CL, is used to focus the beam in one direction so that both polarizations can be measured with a single CCD camera. Note that the Wollaston prism and the resulting separated beams are shown rotated 90° with respect to the actual orientation.

Extracting the THz signal from the camera images. Utilizing a Wollaston prism, a cylindrical lens, and a series of relay lenses, two orthogonally polarized (denoted + and −) components of the elliptically polarized gate beam are imaged onto separate portions of the camera, a) and b). The images of the gate beam are shown with, a), and without, b), the THz beam incident on the ZnTe detection crystal. Images a) and b) are the result of averaging 100 images of individual laser pulses. The difference, c), between image a) and image b) is calculated by simply subtracting the number of counts at each pixel. Oscillations due to water vapor absorption are clearly evident in the difference image c). After vertically summing the top and bottom halves of the difference image, the resultant traces for each polarization component are shown in d) to be very close to mirror images of one another.

Comparison of the single-shot technique with the step scan technique for both LiNbO3 THz generation, a) and b), and ZnTe THz generation, c) and d). These measurements were performed without the cryostats in place. For the LiNbO3 result, a), the THz beam path was not purged of water vapor whereas the THz beam path for the ZnTe result, c), was in a dry nitrogen purged environment. All traces are normalized to their peak value.

Illustration showing the measured THz electric field after passing through a silicon sample in the cryostat system at the peak of the applied magnetic field. The THz radiation in this trace was generated in LiNbO3, and the THz trace is a result of a single-shot measurement. The time scale of the magnetic field pulse is ~109 times slower than the time scale of the THz pulse; therefore, the magnetic field variation during the interaction of the THz radiation with the sample is negligibly small.

Measured THz waveforms for both the LiNbO3, a), and ZnTe, b), generation. At 0 T we measure the transmitted THz waveform with and without optically pumping the silicon sample. At high magnetic field, the silicon sample is optically pumped. Data was taken at 10 K for a) and 83 K for b).

Magnetic field dependence of the relative THz transmission for optically pumped silicon. Data shown in a) and b) were taken with LiNbO3 generation, and the sample temperature was 10 K. Data shown in c) was taken with ZnTe generation, and the sample temperature was 83 K. Cyclotron resonance lines can be seen and the dashed lines are a guide to the eye for the heavier mass feature (blue) and the lighter mass feature (red). All traces are offset linearly with respect to the incremental increase in magnetic field.

Cyclotron resonance center frequency vs. magnetic field. Results from data taken at 10 K and 83 K with both generation schemes is combined to identify two features with frequency linear with applied magnetic field.